I was in TK Max buying some socks and I decided to have a look in the 'boys toys' section as well.
And I found a Death Star!
Well, it was an ice cooler Death Star. It was only five quid.
Here it is, next to my mouse for a bit of scale...
It's in two halves and is made from rubber. That's not so bad, though it is flexible. Fortunately we have a can of expanding foam in the shed so I'm going to fill it with that to make it solid without adding much weight.
For a fiver it's worth a punt as a bit of scenery in the background for a space game or two. Give it a paint job and it should do the trick - not too big and not too small.